The present invention relates optical communications, and, more particularly, to a method for traffic grooming, wavelength assignment and spectrum allocation.
In the ITU-T standardized fixed grid networks [ITU-T], fixed amount of spectrum (50 GHz) is allocated to every channel irrespective of the operating line rate, and the center frequency of a channel remains fixed (FIG. 1(a)). Such a fixed channel grid may not be sufficient to support immerging super-channels which operate at 400 Gb/s or 1 Tb/s line rates. For example, 50 GHz of spectrum is not sufficient for 400 Gb/s and 1 Tb/s channels which require 75 GHz [Y Huang] and 150 GHz [S Gringeri] of spectrum respectively. On the other hand, supporting such super-channels by increasing the channel spacing in fixed grid networks may not optimize the spectrum allocation for channels operating at lower line rates. For example, 10 Gills channel requires only 25 GHz of spectrum. Thus, no single fixed channel grid is optimal for all line rates.
There has been growing research on optical WDM systems that are not limited to fixed ITU-T channel grid, but offers flexible channel grid to increase spectral efficiency. We refer to such gridless WDM networks as Flexible Optical WDM Networks (FWDM). In FWDM networks, flexible amount of spectrum is allocated to each channel, and the channel center frequency may not be fixed (FIG. 1(b)). Thus, while establishing a channel in FWDM networks, control plane must follow (1) the requirement of having the same operating wavelength on all fibers along the route of a channel which is referred to as the wavelength continuity constraint, (2) the requirement of allocating the same amount of spectrum on all fibers along the route of a channel which is referred to as the spectral continuity constraint, and (3) the requirement of allocating non-overlapping spectrum with the neighboring channels in the fiber which is referred to as the spectral continuity constraint. The problem of finding a channel satisfying these constraints is referred to as the routing, wavelength assignment, and spectrum allocation (RWSA) problem.
FWDM networks remove the fixed channel grid restriction and allow non-uniform and dynamic allocation of spectrum. Channels with finer granularity line rates can be supported by using either Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme with variable subcarrier assignment or single carrier modulation schemes with switchable modulation stages. Such channels are referred to as flexible channels in spite of flexible spectrum allocation, spectral utilization of existing channels is limited by the stranded channel capacity in FWDM networks that support discrete sets of line rates. Additionally, there is a requirement of allocating guard bands between channels in order to avoid inter-channel interference. The total required spectrum for guard bands increases in proportion to the number of multiplexed channels in a fiber. For example, supporting 10 individual connections demanding 10 Gb/s line rates per connection using 10 of the 10 Gb/s flexible channels allocate 10 times more spectrum for guard bands compared to supporting the same amount of traffic using a single 100 Gb/s channels, and aggregating all connections in a single channel. An effective solution to improve the utilization of channels and to reduce the wasted spectrum in terms of guard bands is to aggregate low speed connections onto high capacity channels. Such functionality is referred to as traffic grooming. Thus, by packing low granularity of traffic into existing channels, traffic grooming improves the channels resource utilization, and by reducing the number of multiplexed channels in the fiber, traffic grooming improves the spectral utilization.
In traffic grooming, WDM signals at the incoming ports are first demultiplexed into individual wavelength channels using bandwidth variable demultiplexer. Channels carrying transit traffic are switched all-optically using an optical cross-connect (OXC). Low speed connections in wavelength channels are aggregated, separated, and switched by first converting input optical signals into electrical signals using variable rate transponders, performing grooming operations at the client layer using an electrical grooming fabric capable of TDM circuit switching or packet switching, and converting electrical signals back to optical signals using variable rate transponders.
One of the open problems in traffic grooming is: for a given configuration of the optical network in terms of the location of optical nodes and deployed fibers connecting optical nodes, a set of line rates offered by the network and respective spectrum requirement, and the required spectrum of a guard band, the problem is how to find channels and route connections over this channels such that minimum amount of spectrum is required to support the traffic. Finding channels in FWDM networks consists of the sub-problems such as how to determine the operating line rates of channels (line rate selection sub-problem), how to route traffic over channels (traffic routing sub-problem), how to route channels over the network (channel routing sub-problem), how to assign wavelengths to channels (wavelength assignment sub-problem), and how to allocate the required spectrum to channels (spectrum allocation sub-problem). Together the problem is referred to as the traffic grooming, routing, wavelength assignment, and spectrum allocation (GRWSA) problem in traffic grooming capable FWDM networks.
While establishing channels in traffic grooming capable FWDM networks, some additional constraints, such as (1) spectral continuity constraint, which is defined as the allocation of same amount of spectrum on all links along the route, and (2) spectral conflict constraint, which is defined as non-overlapping spectrum allocation to neighboring channels routed though the same fiber, must be maintained in addition to the conventional wavelength continuity constraint, which is defined as the allocation of the same wavelength on all links along the route of a channel.
The traffic grooming, routing, and wavelength assignment (GRWSA) problem in fixed grid networks has been studied in detail by others; however none of the proposed solutions in fixed grid networks is applicable to the GRWSA problem due to additional flexibility in selection of line rates of channels and allocation of spectral resources to channels in FWDM networks.
In a prior work, applicants address the RWSSA problem for optical-layer traffic grooming capable FWDM networks. The RWSSA problem is a special case of the GRWSA problem with additional constraints, such as subcarrier continuity constraint and different wavelengths constraint. The proposed solution in in this prior work by applicants is applicable to the GRWSA problem in FWDM networks that supports channels with only OFDM modulation scheme; however the proposed solution in [IR3] is not applicable to the GRWSA problem for the FWDM networks that supports channels with single carrier modulation schemes.
In another work, the authors simplify the GRWSA problem and formulate the problem as in Integer Linear Program (ILP). That proposed solution is not scalable for the large network size. Additionally, that proposed solution does not solve the wavelength assignment and the spectrum allocation subproblems. That proposed ILP is only applicable to networks that supports only OFDM modulation scheme.
Accordingly, there is a need for an efficient OFDM transceiver that overcomes the shortcomings of prior attempts.